It is known that the wheels of road vehicles often require a balancing operation, which consists in applying to the wheel rim small weights, made of lead or other material, which are suitable to compensate for the uneven distribution of the masses in the tire.
To perform this operation, it is known to use balancing machines that support the wheels to be balanced so that they can rotate about an axis, the machines being provided with electronic means that are suitable to detect any relative imbalances between the two sides of said wheels.
In particular, each balancing machine comprises rectilinear guiding means, which are parallel to the wheel rotation axis and on which an indicator element, provided with an end that acts as a reference and can be inserted in the rim, can slide.
In a first step of the balancing operation, the operator slides the indicator element until said end is alternatively arranged in two reference points chosen inside the rim, so as to define two planes that are perpendicular to the rotation axis and are conventionally termed “compensation planes”.
In a second step, the electronic means of the machine, by turning the wheel, measure the imbalance with respect to the compensation planes and provide, once this measurement has been completed, the values of the weights that must be applied to the wheel at said planes.
In a third step of the balancing operation, the indicator element has to be repositioned at the two reference positions in order to allow the operator to apply the weights to the rim, for example by means of an elastic clamp, which is fitted on the indicator element at the end that acts as a reference.
To perform correct positioning of the indicator element at the positions that were preset in the first step, wheel balancing machines are provided with particular devices that are suitable to detect the current location of said element with respect to the rectilinear guiding means and automatically lock its translational motion when, during the third step, it has been returned to said preset positions.
These devices are commonly provided with rack elements and/or with toothed transmission belts, which are associated with the sliding indicator element and can be locked by means of a conventional detent mechanism, which is operated when one of the preset positions is reached.
These known types of device are not free from drawbacks, including the fact that the means used to lock the indicator element are constructively very complicated and difficult to apply to balancing machines because of their complexity.
Moreover, conventional devices are particularly expensive and bulky and generally require substantially large components, such as for example very long belts.